The present invention relates to apparatus and methods for acting on charged particles. The invention relates in particular, but not exclusively, to a charged particle focusing system using magnetic fields to achieve mass (and energy) dependent focusing of a charged particle beam, or series of charged particle beams, so that very high beam currents of mass analysed charged particles, typically positive ions, can be extracted from a multiple slot source and transmitted without any substantial change in particle current distribution in a plane containing the nominal beam direction and at right angles to the plane of mass dispersion. More specifically, a series of uniform ribbon ion beams can be extracted from a multiple slot ion source and mass analysed to achieve high beam purity without disturbing the uniformity or geometry of the ribbon beam. In other aspects the invention relates to means for using elements analogous to optical elements as part of a system to achieve optimum performance for particular requirements and circumstances; and to means for removing the particle masses not transmitted from the system where their continuous build up might lead to undesirable consequences; and to means for preventing the high currents produced from causing surface charging problems on semiconductor wafers or flat panel display substrates; and to means for achieving the required ion source and extraction conditions necessary for the successful application of this invention.
At the end of this specification there is set out a list of references which will be referred to in this specification to assist understanding of the invention, the contents being incorporated herein by reference. Ions are extracted from ion sources [1] and magnetically analysed to achieve mass separation [2] in order to produce a high purity, directed beam of ions (usually positive ions) which can be used to implant into various substrates, of which semiconductor wafers, solar cells and flat panel displays are important commercial examples. Existing technology predominantly uses a system of analysis which will be referred to as xe2x80x98conventional mass separatorxe2x80x99 optics [3] which restricts the system to the production and analysis of a single ion beam with a significant constraint on the size (and beam current) of that single ion beam.
In U.S. Pat. No. 4,578,589 there is described firstly a conventional apparatus for producing a mass analysed ribbon beam of charged particles in which the ribbon beam is analysed by dispersion in a plane perpendicular to the slot producing the ribbon beam. This is followed by a description of the invention of that prior patent, in which a ribbon beam is mass analysed by dispersion in a plane parallel to the slot producing the ribbon beam. The first known system will now be described briefly with reference to FIGS. 1a to 1c of the present specification, followed by a description of the second form of known apparatus, described with reference to FIGS. 1d to 1e of the present specification.
FIG. 1a shows the dispersion plane (the plane in which there is dispersion of the ion beam into many directions according to their (mass)xc3x97(energy) product and charge state) of a conventional mass separator. The ion beam can be extracted from the ion source [4] as a circular beam, but where a high beam current is required it is usual to extract from a long slot (long in this context being typically a 10:1 aspect ratio or more). The ion source 11 produces ions which are extracted from the ion source aperture 12 (circular or long slot) using electrically biased extraction electrodes 13 to form an ion beam 14 (with an energy determined by the extraction voltage) which typically diverges from the ion source extraction region. The ion beam is then passed between the poles of an analysing magnet 15 as also shown in a side view in FIG. 1b, the beam in this case being a parallel ribbon beam 14A. This magnet has two functions, one being to achieve mass dispersion and the other being to focus the beam so that mass analysis can be achieved at the resolving slit 16. It is necessary to focus through a resolving slit so that slightly lower ion masses (deflected through a larger angle) or slightly higher masses (deflected through a smaller angle) are not transmitted. This analysis technique does not allow the use of multiple ion beams (as viewed in the dispersion plane of FIG. 1a) and the size of the beam in the extraction slot plane is limited by size of the magnet pole gap. The size and therefore the cost of the magnet, and its power consumption (for an electromagnet) are important commercial considerations. One technique that has been used to improve this situation is shown in FIG. 1c. The use of a curved extraction geometry [5] to produce a converging ribbon beam in the plane containing the axis of the long extraction slot (referred to as the xe2x80x98ribbon planexe2x80x99) with a beam crossover in the magnet pole gap, increases the size of the beam which can be transmitted through a particular size pole gap. If, as is usually the case, a parallel beam is required, then an optical element 17 focusing in the ribbon plane is required to produce a parallel beam (such as a curved electrode acceleration system [5]) is required after the resolving slit. Referring again to FIG. 1a, the divergence in the dispersion plane, which is normally small (typically a half-angle of 1-3xc2x0), may be acceptable for ion implantation; if it is not then an optical element 18 in the dispersion plane can be used to create a parallel beam before arrival at the target 19. The optical components 17 and 18 may be separate or achieved in a single optical element.
In order to overcome the current limitations imposed by conventional mass separator optics, the previous invention [6] by the present inventor (in U.S. Pat. No.4,578,589) achieved improvement by placing the long ion source slot in (or parallel to) the dispersion plane. This removed the practical correlation between the length of the slot and the pole gap required in the analysing magnet and made it possible to analyse beams from a series of long slots. FIG. 1d shows the dispersion plane of such a system with a parallel beam 24P (in the dispersion plane) leaving the ion source 21, from a long slot 22 and extraction electrodes 23 and entering the analysing magnet 25, the apparent object position being at infinity. The length of the slot is limited only by the acceptable divergence from the resolving slit 26 (from the point of view of the angular acceptance of the rest of the ion beam system) and the maximum acceptable length from the magnet exit to the resolving slit. Multiple slots, one above the other, see the same geometry in the dispersion plane. FIG. 1e shows a side view along the axis of the long slot, the beamline being unfolded into a single plane for convenience of illustration. A divergent beam 24D is shown leaving the ion source 21, its outlet aperture 22 and the beam forming extraction electrodes 23. The beam enters an angled entry analysing magnet field which (for this particular angle of entry) produces a convergent lens 27A (a well known technique for achieving useful converging or diverging focusing [7]), which significantly reduces the divergence of the beam, and then is focused more at the angled exit region 25B and 27B, ideally producing a near parallel beam. FIG. 1f shows a source-to-magnet view of a multiple (three) beamlet beam 24M from three outlet apertures 22. The direction of each of the beamlets in the extraction region is chosen for optimum transmission through the analysing magnet.
The two mass analysis techniques described above represent the known relevant prior art with regard to overall system design. Other relevant prior art includes the angled entry focusing [7] already mentioned, and magnetic multipole focusing.
Magnetic multipole focusing is commonly used in accelerator beamlines [8], the most commonly used being the quadrupole lens. This lens is shown in FIG. 2. A beam travelling in the direction of the z-axis 30 (normal to the x and y axes) will experience the action of two focusing planes yz 31 and xz 32. One of these planes will be a diverging lens and the other a converging lens, depending on direction of magnetisation, beam direction along the z-axis and particle charge polarity. When two quadrupole lenses are used in combination with alternate magnetic polarity, the overall focusing in both planes can be converging [9]. The important aspect of the prior art use of these lenses is that the general direction of charged particle beam propagation is along the axis 30 of the lens (the z-direction) and is a beam of circular symmetry.
The limitation of the existing prior art technology has always been the high cost associated with producing high beam currents and the situation is becoming increasingly severe as the result of the increasing demand for low energy ion beams. The requirement for low energy tends to lead either to a reduced extraction voltage with a consequent loss of beam current [10], the use of xe2x80x98accel/decelxe2x80x99 extraction as described in the present inventor""s previous patent [6] or separate deceleration of the beam [11] after extraction and before or after magnetic analysis. These techniques are limited by space charge problem limitations and beam aberrations caused by trying to maximise the current density in the beam in order to maximise overall beam current.
These problems can be overcome either by not using analysis at all, or by conceiving an analysis geometry that does not limit the length of the outlet slot of the ion source or does not limit the number of slots. It is generally concluded that the first option (no mass analysis) is not a viable option for integrated circuit technology as the beam purity requirements are too severe. It has been considered to be viable for flat panel display implant, basically on grounds of cost rather than desirability.
The ideal solution to the beam current limitation problem is a mass analysis technique that can be used with any number of any length ion source outlet slots.
According to the present invention in a first main aspect there is provided apparatus for acting upon charged particles in dependence upon one or more parameters comprising mass and/or energy and/or charged state of the particles, comprising an array of elongate magnetic poles extending longitudinally in a direction of elongation of the array; the array having a reference surface extending in the direction of elongation of the array of magnetic poles and passing through the array with at least one magnetic pole on each side of the reference surface; means for providing charged particles entering into or originating in the field of the magnetic pole array; the magnetic poles having a configuration in a plane perpendicular to the said direction of elongation such as to give parameter dependent change of direction to charged particles moving in the magnetic pole array with a direction of movement in or substantially parallel to the reference surface and other than the direction of elongation of the magnetic pole array; whereby parameter dependent selection of charged particles may be achieved by parameter dependent dispersion in a plane transverse to the reference surface.
There may also be provided in accordance with this aspect of the invention apparatus for acting upon charged particles in dependence upon one or more parameters comprising mass and/or energy and/or charged state of the particles, comprising: an array of elongate magnetic poles extending longitudinally in the direction of a longitudinal axis of the array; the array having a reference surface containing or substantially parallel to the longitudinal axis and passing through the array with at least one magnetic pole on each side of the reference surface; means for providing charged particles entering into or originating in the field of the magnetic pole array at a position spaced from the said longitudinal axis; the magnetic poles having a configuration in a plane perpendicular to the said longitudinal axis such as to give parameter dependent change of direction to charged particles moving in the magnetic pole array with a direction of movement in or substantially parallel to the reference surface and other than the direction of the said longitudinal axis of the magnetic pole array; whereby parameter dependent selection of charged particles may be achieved by parameter dependent dispersion in a plane transverse to the reference surface.
In connection with this aspect of the invention, and all other aspects, it is to be appreciated that where features of the invention are set out herein with regard to apparatus according to the invention, such features may also be provided with regard to a method according to the invention, and vice a versa.
Also, it is to be appreciated that where preferred, or essential, features of the invention are set out with regard to the various aspects, any one or more of these features may be provided in accordance with the invention in combination with any one or more other features of that, or other, aspects of the invention.
In particular, in connection with the first aspect of the invention, there is also provided in accordance with the invention a method of acting upon charged particles in dependence upon one or more parameters comprising mass and/or energy and/or charge state of the particles, comprising: providing an array of elongate magnetic poles extending longitudinally in a direction of elongation of the array; the array having a reference surface extending in the direction of elongation of the array and passing through the array with at least one magnetic pole on each side of the reference surface; providing charged particles entering into, or originating in, the field of the magnetic pole array; moving the charged particles in the magnetic pole array with a direction of movement in or substantially parallel to the reference surface and other than the direction of the said direction of elongation of the magnetic pole array; producing a parameter dependent change of direction in the movement of the charged particles by virtue of the magnetic fields produced by the configuration of magnetic poles in a plane perpendicular to the said direction of elongation; and effecting parameter dependent selection of charged particles by parameter dependent dispersion in a plane transverse to the said reference surface.
It is to be appreciated that the reference surface consists of a virtual surface passing through the array of magnetic poles and defined for the purposes of setting out the features of the present invention. The said references surface is not limited to a surface of a physical object. In all aspect of the invention, it may be preferred to arrange that the array of elongate magnetic poles has a longitudinal axis defining the said direction of elongation of the array of magnetic poles. The said longitudinal axis of the array may be contained in the reference surface or may be spaced from but parallel to the reference surface.
Many variations of the invention in this aspect may be provided. The charged particles moving in the magnetic pole array may consist of a beam of charged particles passing through the magnetic pole array, or the magnetic particles may originate in an area within the magnetic pole array. The most usual use of the parameter dependent selection will be mass analysis for example for producing a selected ion beam for ion implantation, or for use in a mass spectrometer or mass separator.
The longitudinal axis of the magnetic pole array may be a rectilinear longitudinal axis, or a curved longitudinal axis. For example the elongate magnetic poles may be straight or curved. The longitudinal axis may constitute all or part of a circle, or other curve. Similarly, the reference surface may be a plane, that is to say a flat surface, or alternatively may be a curved surface for example a part spherical or cylindrical surface.
The configuration of magnetic poles in a plane perpendicular to the longitudinal axis may have a geometric symmetry on either side of the reference surface, that is to say the symmetry of the physical components may be symmetrical about the reference surface,. even though the magnetic orientation of the poles may or may not be symmetrical. In such a case the reference surface forms a reference surface of geometric symmetry of the array.
In particular preferred forms, the array of magnetic poles is such as to provide between opposed poles an extended region of magnetic field in which the charged magnetic particles pass with a curved motion imposed thereon by the field, together with entry and exit regions which provide curved magnetic fields giving focusing and/or divergence of a beam of charged particles, together with parameter dependent dispersion of the charged particles in the said plane transverse to the reference surface. By parameter dependent dispersion is meant the different changes of direction of movement produced by the magnetic pole array on particles having different parameters. Most usually in embodiments of the invention, it is arranged that the particles of a beam are focused by the effect of the magnetic pole array, and the parameter dependent dispersion produces focusing of the beam of particles at different focal points along the general direction of propagation of the beam. In such an arrangement, preferably one or more barriers are provided giving an analysis aperture or apertures at the focal point of a desired species in the particle beam, preferably the barrier or barriers be aligned along the general direction of propagation of the beam.
In particular preferred form, it may be arranged that the array of magnetic poles is such as to provide between opposed poles an extended region of magnetic field in which charged particles moving with a direction of movement in or substantially parallel to the reference surface have a curved motion imposed thereon by the field, together with entry and exit regions which provide curved magnetic fields, curved in a plane perpendicular to the reference surface, giving focusing or divergence of a beam of charged particles passing through the curved field at an angle to the normal to the entry or exit region. In some embodiments the extended region also has curved magnetic fields curved in a plane perpendicular to the reference surface.
Usually in embodiments of the invention the array of magnetic poles, also referred to as a multiple magnetic pole array, or multipole, comprises an array of magnetic poles distributed in a plane at right angles to the longitudinal axis (which may be an axis of symmetry), with a geometry appropriate to the focusing requirements and extended along the straight or curved axial direction of the magnets a distance determined by the focusing requirements, the geometry and strength of these poles being consistent along this multipole axis. There is therefore no dispersion in a plane containing the multipole axis. When a multipole is used in its traditional way, with the nominal beam direction along the axis, the arrangement of poles is usually one of poles of alternate polarity distributed around the arc of a circle. In preferred forms of this invention, a series of poles along two straight or curved lines with a plane of geometric symmetry between them will be the most common geometry (but not restricted to this geometry) with the nominal beam direction in this plane of geometric symmetry. Opposite poles across this plane of geometric symmetry can either be similar or opposite direction of magnetisation, giving a totally different kind of lens action. The simplest form of multipole lens action is a dipole with poles of similar direction of magnetisation and an extended region of magnetic field between them so that the beam is deflected as it passes through the dipole (angled entry and exit fringe field focusing). The prior art has its dispersion plane at the plane of geometric symmetry of a variable strength dipole (wedge shaped magnet); in preferred embodiments of this aspect of the invention there is no dispersion in this plane because the dipole has consistent properties along its length.
The magnetic poles can be produced electromagnetically or they can be permanent magnets.
The important aspect in preferred forms of the invention is that it is mass dependent and is used as a method of beam analysis. The term xe2x80x98mass dependentxe2x80x99 is a simplification of the true situation. The path of the charged particle is dependent not only on mass but also upon the energy and charge state of the particle. An ion beam extracted from an ion source will have an energy determined by the acceleration voltage and the charge state of the ion species.
According to the present invention in a second main aspect there is provided apparatus for selection of particles of a beam of charged particles in dependence upon one or more parameters comprising mass and/or energy and/or charged state of the particles, comprising an array of elongate magnetic poles extending longitudinally in a direction of elongation of the array; the array having a reference surface extending in the direction of elongation and passing through the array with at least one magnetic pole on each side of the reference surface; the magnetic poles having a configuration in a plane perpendicular to the said direction of elongation such as to give parameter dependent focusing of a charged particle beam or series of beams passing through the magnetic pole array with a general direction of propagation in or substantially parallel to the reference surface and other than the direction of elongation of the magnetic pole array; the said reference surface being a surface of geometric symmetry with regard to the array of magnetic poles, and the configuration of the magnetic poles being such as to achieve analysis by using parameter dependent dispersion and focusing in a plane at right angles to the surface of geometric symmetry.
Preferably the surface of geometric symmetry will be straight in the general direction of propagation of the charged particle beam but can be curved in a plane at right angles to this direction.
According to the present invention in a third main aspect there is provided apparatus for selection of particles of a beam of charged particles in dependence upon one or more parameters comprising mass and/or energy and/or charged state of the particles, comprising an array of elongate magnetic poles extending longitudinally in a direction of elongation of the array; the array having a reference surface extending in the direction of elongation of the array and passing through the array with at least one magnetic pole on each side of the reference surface; the magnetic poles having a configuration in a plane perpendicular to the said direction of elongation such as to give parameter dependent change of direction to charged particles moving in the magnetic pole array with a direction of movement in or substantially parallel to the reference surface and other than the direction of elongation of the magnetic pole array; the configuration being such as to permit passage through the array of a beam of charged particles having an elongate cross section perpendicular to the general direction or directions of propagation of the beam, the elongate cross section being elongate in a direction lying in or parallel to the said reference surface; whereby parameter dependent selection of charged particles may be achieved by parameter dependent dispersion in a plane transverse to the reference surface.
The elongate cross-section, charged particle beam, with substantial extension across the reference surface, may be provided by what is normally termed a xe2x80x98ribbonxe2x80x99 beam. The ribbon beam can be curved in a direction at right angles to the general direction of propagation of the beam (a curved ribbon). A uniform ribbon beam is propagated through the lens system as a uniform ribbon beam. The dispersion characteristics of this invention in this form differ from conventional mass separator optics in that the analysing magnetic fields do not have a predominant component parallel to the axis of the long extraction outlet slot (and this therefore avoids the conflict between slot length and magnet pole gap) and the dispersion plane is not parallel to the surface of geometric symmetry between the two poles but is similar in that the dispersion plane can be at right angles to the slot. The dispersion plane for this aspect of the invention is never in the plane containing the slot, and this distinguishes it from the other prior art analysis technique which is defined by the fact that the dispersion plane is in the plane of the extraction outlet slot.
According to the present invention in a fourth main aspect there is provided apparatus for acting upon charged particles in dependence upon one or more parameters comprising mass and/or energy and/or charged state of the particles, comprising an array of elongate magnetic poles extending longitudinally in the direction of a longitudinal axis of the array; the array having a reference surface containing the longitudinal axis and passing through the array with an equal number of magnetic poles on each side of the reference surface the opposing poles having a common direction of magnetisation perpendicular to the reference surface; the array of magnetic poles being such as to provide between opposed poles extended regions of magnetic field in which the charged particles pass with a curved motion imposed thereon by the magnetic field, together with entry and exit regions which provide curved magnetic fields giving parameter dependent dispersion of the charged particles in a plane transverse to the reference surface; the magnetic pole array having an initial extended region of magnetic field, one or more intermediate regions, and a final extended region of magnetic field; and the apparatus having means for providing charged particles entering into, or originating in, the initial extended region; the magnetic poles having a configuration in a plane perpendicular to the said direction of elongation such as to give parameter dependent change of direction to charged particles moving from the initial extended region of magnetic field with directions of movement in or substantially parallel to the reference surface and other than the direction of elongation of the magnetic pole array, leaving this extended region of magnetic field at an angle to the normal from the longitudinal axis of the array, passing through the intermediate regions of the array, and moving into the final extended region of magnetic field of opposite magnetic polarity to the initial region; whereby parameter dependent selection of charged particles may be achieved by parameter dependent dispersion in a plane transverse to the reference surface.
The significant feature of this aspect of the invention when the particles originate in the initial extended region of uniform magnetic field, is that the general direction of the charged particle beam is continually changing direction as it passes through the extended regions of magnetic field and, to a lesser extent, as it passes through the multipole region. The concept of a singular xe2x80x98general directionxe2x80x99 for the beam as a whole is therefore not appropriate. The condition that the beam passes through the pole array xe2x80x98at an angle to the normal from the straight or curved axis of the pole arrayxe2x80x99 is a necessary condition for strong focusing in the same way that angled entry into a fringe field (as will be explained hereinafter) is a necessary condition for convergent or divergent focusing. Some weak second order focusing will occur even for beams not at an angle to the normal from the multipole axis simply because the beam is continuously changing direction.
The concept of a xe2x80x98general direction of propagationxe2x80x99 for the beam is appropriate when the beam starts in magnetic field free space. Thus in accordance with this fourth aspect, the charged particles may, for example, enter into the initial extended region as a beam of particles, or the particles may be generated in the initial extended region of field. Also, the particles moving in the final extended region may exit the region and then pass to other components, or the particles may be used within the final extended region, for example for ion implantation in a target in the final extended region.
The significance of the extended field regions between the first set of poles (the entry poles) is that it creates the direction of travel through the multiple magnetic pole array that is necessary to create strong focusing (assuming that the beam entering this extended field region did not originally have this direction of travel) and determines, together with magnetic field strength in the multipole region, the strength of the lens action (for a charged particle with a particular mass, energy and charge state) The exit extended field determines the angle of the beam when it leaves the magnetic field region.
A preferred feature for use with the fourth aspect of the invention, is that the initial and final extended regions of magnetic field are substantially symmetrical about a plane perpendicular to the reference surface. This plane is preferably positioned equidistant between the initial and final extended regions.
This special case is important because a uniform, parallel ribbon beam entering the mass analysing system leaves as a uniform, parallel, analysed beam travelling in the same direction. The condition that the entry and exit fields are equal in magnitude can mean either equal flux, or in a desirable special case equal in flux density distribution and geometry (exit field is a mirror image of the entry field). The latter condition allows the use of complex pole shapes and magnetic fields that are near to saturating the magnetic material of the poles and, because of the intrinsic symmetry of the system, maintains the parallel in/parallel out characteristic. This magnetic field arrangement is intrinsically balanced and this has a number of practical advantages.
The main aspects of the invention not only include the facility of mass analysis but also the facilities of energy analysis and charge state analysis. The initial charge state of the particle determines the energy for a given acceleration voltage. The charge state can change during transmission (by interaction with neutral gas molecules, for example), the change from singly charged to neutral being particularly important for ions. This can lead to a number of different particle energies in the beam after subsequent electrostatic acceleration. For low energy semiconductor implant, for example, deceleration of the beam may be desirable, Any neutrals in the beam will not be decelerated and the high energy ion impurity in the beam are extremely undesirable. Magnetic analysis is sensitive to mass, energy and charge state according to the equation:
R=143.95SQR(MV/e)/B
where R is the radius of the circular movement of the ion in the magnetic field, B is the magnetic flux density in gauss, M is the ion mass in amu, V is the acceleration voltage and e is the charge state.
It is thus possible to use a multipole lens before or after an acceleration or deceleration stage to filter out ions with unwanted charge states (particularly before acceleration or deceleration) or unwanted energies (particularly after acceleration or deceleration)
According to the present invention in a fifth main aspect there is provided apparatus for acting upon charged particles in dependence upon one or more parameters comprising mass and/or energy and/or charged state of the particles, comprising an array of four elongate magnetic poles extending longitudinally in the direction of a longitudinal axis of the array; the array having a reference surface containing the longitudinal axis and passing through the array with two magnetic poles on each side of the reference surface; the magnetic poles having a configuration in a plane perpendicular to the said longitudinal axis to give a first extended region of substantially uniform magnetic field between a first pair of opposing poles with a direction of magnetisation perpendicular to the reference plane, and a second extended region of substantially uniform magnetic field between the other two opposing poles with an opposite direction of magnetisation, the region between these two sets of poles forming a quadrupole magnetic field region; the magnetic poles having a configuration in a plane perpendicular to the said longitudinal axis such as to give parameter dependent change of direction to charged particles moving in the magnetic pole array with a direction of movement in or substantially parallel to the reference surface and other than the direction of the said longitudinal axis of the magnetic pole array; the charged particles moving in a parameter dependent curved trajectory in the first magnetic field, high curvature trajectories not reaching the quadrupole field region, low curvature trajectories passing through the quadrupole field region and a particular parameter dependent trajectory passing into and along the quadrupole field region; whereby parameter dependent collection of charged particles may be achieved by placing collector means on the quadrupole axis.
This fifth aspect is a very high resolution technique for collecting a particular mass species for mass separation or mass spectrometry. It would not be suitable for beam formation for ion implantation because of the uncertain optics of the beam as it travels along the quadrupole axis. It can be part (or all) of an ion beam system, the lens being used to analyse and deliver an ion beam to target but with the added facility to analyse the content of the beam passing through this lens.
It is possible to have a situation where the bean is xe2x80x98deflected fromxe2x80x99 the multipole. This only applies when the entry poles are of similar direction of magnetisation and this entry pole field is strong enough and extensive enough to turn a beam of a particular mass, energy and charge state parallel to the multipole axis before it reaches a field direction in the multipole that changes the deflection direction. In the limiting case a beam can reach a condition where it travels along the multipole axial direction. A very small increase in mass would lead to the beam being transmitted into the multipole region and a small decrease would cause the particle to approach and then turn away from the multipole region, experiencing some mass dependent focusing while in the multipole region.
According to the present invention in a sixth main aspect there is provided apparatus for acting upon charged particles in dependence upon one or more parameters comprising mass and/or energy and/or charged state of the particles, comprising an array of elongate magnetic poles extending longitudinally in a direction of elongation of the array; the array having a reference surface extending in the direction of elongation of the array of magnetic poles and passing through the array with at least one magnetic pole on each side of the reference surface; the array of magnetic poles being such as to provide between opposed poles extended regions of magnetic field in which the charged particles pass with a curved motion imposed thereon by the magnetic field, together with entry and exit regions which provide curved magnetic fields giving parameter dependent dispersion of the charged particles in a plane transverse to the reference surface; the magnetic poles having a configuration in a plane perpendicular to the said direction of elongation to give a extended region of magnetic field between two poles with a direction of magnetisation perpendicular to the reference surface; the magnetic poles having a configuration in a plane perpendicular to the said direction of elongation such as to give parameter dependent change of direction to charged particles moving in the magnetic pole array with a direction of movement in or substantially parallel to the reference surface and other than the direction of elongation of the magnetic pole array; the beam or beams moving in a parameter dependent curved trajectory in the extended magnetic field, high curvature trajectories staying within the extended field region, low curvature trajectories passing through the extended field region, whereby parameter dependent separation of beams by reflection of high curvature beam trajectories and transmission or collection of low curvature beam trajectories, the extended field region acting as a selective reflection mirror.
The most useful application of this aspect is for a beam entering the multipole extended entry region at an angle, say 45xc2x0, and reflecting out to give a reflection angle of 90xc2x0. This reflects masses lower than a certain value and transmits or collects higher masses. If the beam geometry allows, it may be possible to use the mass dependent focusing of reflected beams to achieve analysis. For the reflection application, the multipole need only be a dipole (or quadrupole).
The optics described in the aspects above provide the required mass dependent properties necessary to achieve a mass analysed beam which can be delivered to a target with the required characteristics. The techniques necessary to utilise the invention include removal of unwanted mass species and beam formation with the required optics. The invention has been described as a single multipole lens structure; the number of poles in the multipole is an important consideration as is the number of lenses that might be used to achieve optimum characteristics.
Preferably, the array of magnetic poles is positioned to act on an ion implantation beam entering the array in a non vertical direction, and arranged to deflect the beam so as to exit the said field in a substantially vertical direction, for ion implantation into a subsequent substantially horizontal target. This arrangement finds particular use for ion implantation into a horizontal or approximately horizontal target, for example being moved on a horizontal or substantially horizontal conveyor, where it is desired to implant by a vertical or near vertical particle beam. When this is arranged conventionally, it is normally necessary to provide components for generating the vertical beam, positioned directly above the moving conveyor belt of targets. This gives the disadvantage of particulates falling onto the wafers with consequent contamination. The embodiment that the invention allows generation of the implantation beam in a horizontal or near horizontal plane, and the reflection or deflection of the beam through a required angle to emerge substantially vertical.
According to the present invention in a seventh main aspect there is provided apparatus for selection of particles of a beam of charged particles in dependence upon one or more parameters comprising mass and/or energy and/or charged state of the particles, comprising an array of elongate magnetic poles extending longitudinally in a direction of elongation of the array; the array having a reference surface extending in the direction of elongation of the array of magnetic poles and passing through the array with at least one magnetic pole on each side of the reference surface; the magnetic poles having a configuration in a plane perpendicular to the said direction of elongation such as to give parameter dependent focusing of a charged particle beam or series of beams passing through the magnetic pole array with a general direction of propagation in or substantially parallel to the reference surface and other than the direction of elongation of the magnetic pole array; the configuration of the magnetic pole array being such as to produce focusing to a cross-over the position of which is parameter dependent, and there being provided a resolving structure allowing transmission of particles of a required species through an aperture positioned at the cross-over.
This aspect of the invention covers the general case of a beam which has a general direction of propagation parallel to, but not necessarily along, the plane of geometric symmetry, and coming to a crossover (focus) at a position not necessarily in the plane of geometric symmetry.
According to a preferred feature for use in the seventh main aspect of the invention, the said reference surface is a surface of geometric symmetry with regard to the array of magnetic poles, and the aperture of the resolving structure lies on the plane of geometric symmetry.
This aspect covers the important practical embodiment where a beam diverging from a source extraction region has an axis of symmetry regarded as the general direction of propagation which coincides with the plane of symmetry of the lens structure (the central plane), or a parallel beam (not necessarily symmetrical about the central plane or, in an important practical embodiment, completely to one side of it) which comes from an object position at infinity. The crossover (focus) occurs in the lens plane. The first of these two geometries has the resolving structure in the middle of the beam and therefore the central part of the beam is lost. The structure therefore needs to be as thin as possible, relying on techniques such as tensioning to keep these structures straight. The presence of the resolving structure as an electrode can be an advantage for high perveance (high space charge) beams as it creates a central region of zero (ground) potential and allows the injection of secondary electrons (to reduce space charge) into the centre of the beam in the extraction region (see a later aspect of this invention). It is important to note that space charge is, in general, not a problem in this invention because the beams are space charge neutralised because there are no electric fields in the lens region and because the invention allows the use of relatively low current density beams. This is due to the lack of restriction on the size and number of beams transmitted from source to target.
The resolving structure allows transmission of the beam that passes through the aperture, usually a slit, and removes any beams which have crossovers at other positions along the resolving structure. Very high mass beams may have crossovers a considerable distance along the lens plane, or no crossover at all (the diverging beam from the extraction region remaining diverging or becoming, in the limiting case, parallel. These high mass beams must be removed without impeding the transmission of the required mass beam.
According to a preferred feature there is provided a transmission limiting structure positioned in a plane transverse to the general direction of propagation of a focused beam through the resolving structure, for preventing transmission of particles following trajectories beyond the range obstructed by the said resolving structure.
This technique would not appear to remove diverging high mass beams that can pass through the second aperture. It will be shown that, provided the geometry of the resolving structure relative to the second aperture is correctly chosen, complete removal of high mass beams can be achieved. In certain circumstances, particularly when beams are narrow (low divergence), the second transmission limiting aperture is used without the resolving structure. Examples of this are the use of a narrow slit to improve the quality of the beam transmitted to further lenses by removal of poor quality beam that is not focused through the slit and the reduction of the maximum divergence of unwanted beams which helps with subsequent mass resolution.
It has been mentioned that part of the resolving structure can be extended along the central axis of the beam towards the source into a magnetic field free region away from the lens entry region, where secondary electrons can be transmitted up through the centre of the beam towards the extraction region, reducing the space charge potential at the centre of the beam. This will reduce the divergence of high perveance (high space charge) beams, particularly important for low energy implantation.
According to the present invention in an eighth main aspect there is provided an extraction assembly for extracting positively charged particles from an elongate charged particle source which is elongate in a direction transverse to the general direction of extraction of the particles, the extraction assembly having an accelerating region followed by a decelerating region to produce a charged particle beam the optics of which are significantly influenced by the space charge of the extracted charged particles; the assembly including an elongate element of conducting material at a floating or a controlled potential, situated at the centre of the beam and parallel to the elongate axis of the source region, and positioned in the electrostatically decelerating field for positive ions or in a field free region; the arrangement being such that the presence of secondary electrons produced by charged particles striking the elongate element, and the presence of the element acting as an electrode, combine to reduce the space charge at the centre of the beam, thus increasing the beam current that can be usefully extracted.
The wire or strip in this aspect could be tensioned to keep it straight, particularly when, in order to minimise beam loss, the wire or strip are very thin. This raises the problem of erosion due to sputtering. In other aspects of this invention, surfaces are struck by unwanted charged particles, particularly in the form of atomic and molecular ions, which can result in undesirable erosion, build up of surface layers and flake formation for example. It would be desirable, particularly when, as in this invention, these regions are very close to the required beam, for the material being struck by these particles to be continuously, and preferably automatically, replaced.
According to the present invention in a ninth main aspect there is provided apparatus for producing or acting upon a beam of charged particles having an elongate cross-section perpendicular to the general direction or directions of propagation of the beam, in which the apparatus includes an elongate element aligned along the elongate axis of the beam which is used to intercept charged particle beams, either to remove those beams or to otherwise influence the behaviour of the charged particle beam, and which is thereby subject to deterioration by contact with the charged particles, including means for moving the elongate element in the direction of its elongate axis to replace the parts thereof which have deteriorated due to contact with the charged particles. The apparatus may comprise a moving wire or strip, which is used to intercept charged particle beams, either to remove those beams or to otherwise influence the behaviour of the charged particle beam, the removal process preventing excessive erosion, that might lead to breakage or ineffectiveness, or preventing surface material build up or flake formation which might lead to undesirable effects on the beam or any process carried out elsewhere in the system.
This aspect can be extended to include tensioning of the wire to keep it straight. The tensioning of components such as resolving and transmission slit components can be regarded as an inventive aspect in itself, this being particularly important when dealing with large ribbon beams.
According to the present invention in a tenth main aspect there is provided apparatus for producing or acting upon a beam of charged particles having an elongate cross-section perpendicular to the general direction or directions of propagation of the beam, in which the apparatus includes an elongate element aligned along the elongate axis of the beam which is used to intercept or otherwise influence the behaviour of the charged particle beam, the apparatus including means for tensioning the elongate element to keep it straight.
The moving and tensioning of a wire or wires or a strip or strips are a further inventive combination.
The important aspects of the invention considered so far are concerned with analysis rather than beam formation. These multipole lenses are multipurpose line lenses for use in an optical system and can be used to focus or make parallel beams for entry into another optical element or for delivery to a target. As the focusing is always mass dependent there is a desirable tendency for successive focusing operations to increase the resolving power of the system. The resolving power of the individual lens/resolving slit combinations is a variable depending on geometric factors.
The simplest example of combined mass analysis and beam formation is the use of a strong multipole lens. A lens can be used, for example, to focus a divergent beam to a converging beam or a parallel beam. In the former case, the focus to a crossover can be used for mass analysis at a resolving slit but the resulting divergent beam after analysis is probably not the ultimate requirement. For most ion implantation processes the ideal beam is a parallel beam.
For simplicity, multipole lenses of the type described in previous aspects of this invention are now going to be simply described as xe2x80x98line lensesxe2x80x99.
A strong line lens is regarded as a lens where a crossover is achieved within the lens region and the beam continues to be focused in the same lens after the crossover. This means it is possible to analyse and then focus to parallel using a single line lens. There is therefore an important specific further aspect of the previously mentioned tenth main aspect.
According to a preferred feature, the position of the crossover is chosen so that subsequent further focusing of the beam transmitted through the resolving slit in the multiple magnetic pole array leads to the production of a beam with required optical characteristics.
The most likely xe2x80x98required optical characteristicxe2x80x99 is a parallel beam. This is a particularly useful line lens geometry when the parallel beam is delivered directly to the target. The quality of the beam leaving the lens is not as good as a xe2x80x98normalxe2x80x99 lens but the mass dispersion is superior. This must be taken into consideration when using a strong lens in a multiple lens optical system.
The two types of line lens from the point of view of the pole arrangement across the plane of geometric symmetry will be called:
xe2x80x98transverse-fieldxe2x80x99 line lenses where the magnetic field between two geometrically opposed poles crosses the plane of geometric symmetry.
xe2x80x98axial-fieldxe2x80x99line lenses where the magnetic field between two geometrically opposed poles passes along the plane of geometric symmetry.
Transverse-field line lenses are strong lenses because the closeness to the reflection condition produces a strong interaction with the multipole fields and creates an enhanced mass dispersion. Transverse-field lenses are in general much stronger than axial-field lenses for a given electrical conductor power consumption.
The aspects of the invention considered have now included both mass analysis and beam optics from a single line lens. There are many reasons for using more than one lens:
a) the quality of the beam (measured by its emittance [1]) determines the resolving power [12] that can be achieved. The emittance can be improved by passing the beam through a narrow transmission limiting aperture or, even better, a combination of both transmission limiting and resolving apertures. A second stage of focusing can then exhibit enhanced resolving power due to the improved quality of the input beam and the cumulative effect of double focusing;
b) a two lens optical system allows multiple crossovers which lead to very good xe2x80x98no-line-of-sightxe2x80x99 characteristics between the extraction region and the first lens mass analysis region, both of which create a significant amount of sputtered material produced by ion bombardment, and the target. Mass analysis is required because of semiconductor sensitivity to impurities and it is therefore necessary to take equally stringent precautions to prevent other forms of contamination from reaching the target;
c) the output optics from the magnetic line lenses may be determined by the needs of subsequent acceleration or deceleration stages. Acceleration is a naturally convergent focusing process so therefore a diverging beam is desirable. Deceleration of high perveance beams is dominated by space charge beam blow-up considerations, and a converging input may be required. If channelling issues are important in a single crystal silicon implant, then an accurately parallel beam may be needed. This may favour the use of xe2x80x98normalxe2x80x99 lenses;
d) when an ion beam contains a substantial fraction of an atomic or molecular species which is considerably lighter than the required species which is to be transmitted through the analysing system, it may be desirable for the first lens to be a weak lens dedicated to the removal of this species. This issue is only significant for transverse field line lenses where such a light ion may be reflected back to the extraction region.
According to a preferred feature there is provided, one or more further arrays of elongate magnetic poles and associated resolving structures arranged to produce parameter dependent focusing of the beam exiting the first array, whereby a combination of components of the arrays obscures line of sight through the combined system for contamination particles due to sputtering or otherwise.
There are a wide variety of ways of using two or more line lenses to produce particular optical requirements. In general, the first lens needs to be independently controllable (because the object position is determined by the extraction conditions) adjusted to focus the required beam through the first aperture. Subsequent lenses do not have to be independently controlled. Doublet and triplet lenses can simplify power supply requirements by using a common power supply, reducing the number of electrical conductors needed and reducing the length of the lens system.
An example of the use of a three lens system consisting of a independent first lens followed by a doublet, introduces another aspect of the use of multiple lenses where the axis of the first lens does not coincide with the axis of the following doublet. The advantage of this asymmetric approach is that it can avoid the need for a resolving plane in the centre of the beam. This is particularly important for low perveance beams, where there is not an extraction divergence problem caused by space charge beam blow-up, which may lead to very low divergence beams. The simplest example is for beam focused to a parallel beam by the first lens; this parallel beam passing into the first lens of the doublet off-centre with the lens power selected to give a parallel beam-crossover-parallel beam geometry with the beam now off-centre on the other side of the doublet plane of geometric symmetry; followed by a repeat process to return the parallel output beam to the first side of the plane of geometric symmetry. This beam geometry gives good resolving power and excellent line-of-sight characteristics.
According to the present invention in an eleventh main aspect there is provided apparatus for selection of particles of a beam of charged particles in dependence upon one or more parameters comprising mass and/or energy and/or charged state of the particles, comprising a first array of elongate magnetic poles extending longitudinally in a first direction of elongation of the array, the array having a first reference surface extending in the first direction of elongation and passing through the array with at least two magnetic poles on each side of the reference surface; the magnetic poles having a configuration in a plane perpendicular to the said first direction of elongation such as to give parameter dependent focusing of a charged particle beam or series of beams passing through the magnetic pole array with a general direction of propagation in or substantially parallel to the reference surface and other than the direction of the said first direction of elongation of the magnetic pole array; the said first reference surface being a surface of geometric symmetry with regard to the first array of magnetic poles, and the configuration of the magnetic poles being such as to produce focusing to a parallel beam exiting the first array; a second array of elongate magnetic poles extending longitudinally in a second direction of elongation of the second array, the array having a second reference surface extending in the second direction of elongation and passing through the array with at least two magnetic poles on each side of the reference surface, the second reference surface being parallel to and spaced from the first reference surface, and the said parallel beam exiting the first array being introduced into the second array off set from the second reference plane; the magnetic poles of the second array having a configuration in a plane perpendicular to the second direction of elongation such as to give parameter dependent focusing of the parallel beam passing through the second magnetic pole array with a general direction of propagation in or substantially parallel to the second reference surface and other than the direction of the said second direction of elongation of the second magnetic pole array; the said second reference surface being a surface of geometric symmetry with regard to the second array of magnetic poles, and the configuration of the second magnetic pole array being such as to produce focusing to a parameter dependent cross-over with the beam emerging from the cross-over on the opposite side of the second reference surface; there being provided a resolving structure at the cross-over defining a resolving aperture by components positioned before and after the cross-over and off set from the second reference surface to the side thereof opposite to the beam.
There may be provided apparatus comprising two or more multiple magnetic pole array line lenses with different, but parallel, planes of symmetry, giving mass dependent focusing of a charged particle beam or series of beams, or a beam with substantial extension across the plane of symmetry, with a general direction of propagation parallel to the planes of symmetry through the multiple magnetic pole line lenses, enabling mass analysis to be achieved by first focusing to a parallel beam, this parallel beam then being introduced off-centre on one side of the plane of symmetry of the next lens or lenses, and then focusing to one or more crossovers in this plane of symmetry, and there being a resolving structure on the opposite side of the plane of symmetry to the beam, allowing transmission of a beam with crossovers at the positions of one or more apertures in this structure.
One characteristic of this technique is that the resolving structure can be xe2x80x98infinitely thinxe2x80x99 because the resolving structure is either on one side or the other of the plane of geometric symmetry.
The next, and most important, general aspect of this invention is the fact that, as the ribbon beam travels along the plane of symmetry of the analysing system, there can be as many beams as is necessary to achieve required beam current. The magnetic circuits will be in series for transverse-field lenses and will have an alternate parallel/anti-parallel structure for axial-field lenses.
According to a preferred feature of the present invention there may be provided a plurality of beam systems with regularly spaced respective reference surfaces each sharing at least part of a common magnetic circuit with neighbouring systems.
The combination of this aspect of the invention with previous aspects results in a fully analysed beamline system where there is effectively no practical limit to the beam current available.
According to the present invention in a twelfth main aspect there is provided apparatus comprising: means for producing an ion beam which enters a target region for ion implantation or other reaction with a target element; wherein the target region is surrounded by a multipole magnetic containment which retains charged particles formed within the target region by reaction with the target, or reaction with background gas or vapour or by other plasma generation means, and allows the beam to pass through the regions between or through parallel sets of cusps; the arrangement being such that an electrically neutral gaseous plasma can be formed or retained in the target region for the neutralisation of surface charge of either positive or negative polarity on the surface of the target element situated in, passing through or being mechanically scanned in the target region.
The previous main aspects of the invention allow very large ion beam currents to be delivered to a target such as a semiconductor wafer or flat panel display substrate which are likely to give rise to surface charging problems. It is very important to this invention that a means of preventing surface charging should be available.
According to the present invention in a thirteenth. main aspect there is provided apparatus for extracting charged particles from an elongate charged particle source which is elongate in a direction transverse to the general direction of extraction of the particles, the apparatus including: means for. providing an electric extraction field formed from two electric field components produced by electrode structures positioned one on each side of a plane containing the elongate axis of the elongate source; means for moving the electric extraction field towards and away from the elongate source; and means for producing relative movement between the two electric field components of the extraction field.
This aspect of the invention relates to the need for the extraction field geometry to optimised for a wide range of extraction voltages. The conventional way to achieve this is simply to move the extraction electrodes away from the source region at extraction high voltages in order to prevent electrical breakdown across excessively high field gradient extraction gaps, and to move the extraction electrode assembly closer to the source at low extraction voltages in order to maintain as high a field as possible in order to maximise the extracted beam current. This simple electrode movement is not ideal; the width of the aperture in the extraction electrode should ideally decrease as the electrode moves closer to the source and increase as the electrode is moved further away.
There are two ways of achieving this. The first is the individual mechanical movement of two independent halves of the electrode structure for both extraction optics control and beam alignment (correcting for small unintentional misalignment which causes the beam the be extracted in other than the required direction). The other technique is to have an array of stationary electrodes and to move the electric potential distribution along these electrodes to create a variable extraction field geometry and to vary the field across these electrodes (i.e. on either side of the beam) in order to achieve an alignment correction function.